This invention relates generally to fission reactors and is particularly directed to a system for controlling the reaction rate, or reactivity, of a small fission reactor.
The typical nuclear fission reactor employs a core having fuel and control elements located therein according to a specific matrix. The control elements are raised and lowered relative to the fuel elements in controlling the fission reaction rate. In most circumstances, with the control elements fully inserted into the core the fission reaction is at its minimum, or lowest, rate or the reactor is shut down. With the control elements withdrawn from the core, or positioned beyond a predetermined location within the core, the fission reaction is allowed to progress at its maximum rate. By varying the position of the control elements, which are typically in the form of rods, the reactivity may be controlled. The control elements are generally comprised of boron or some other "poison" material which absorbs the fast neutrons emitted from the fuel elements which are necessary for sustaining the fission reaction.
In most liquid metal fast breeder reactors the entire core is submerged in a pool of coolant which is typically liquid sodium. The sodium is forced through the passages of the core and over the fuel and control elements therein in order to remove the heat generated in the fission reaction. An intermediate sodium-to-sodium heat exchanger generally is used to take the heat from the primary sodium coolant and transfer it to a separate supply of secondary sodium coolant which is then directed via a closed loop through a water-to-sodium heat exchanger wherein steam is generated. The steam is allowed to expand through turbines which, in combination with other power conversion apparatus, is used for driving generators for producing electrical energy.
The control elements are generally located within the reactor vessel and in direct contact with the sodium coolant. This arrangement limits accessibility to the control elements for maintenance and replacement purposes. The liquid sodium coolant is chemically reactive and thus necessary precautions typically involving complicated procedures must be taken to eliminate oxidation during the inspection and replacement of the control elements. In addition, the small size of the typical liquid metal fast breeder reactor core imposes restrictions on its control system. For example, because the fuel core of a fast breeder reactor is constructed with a comparatively compact arrangement of fuel of high concentration and is thus small in size compared to that of thermal reactors, a closely spaced arrangement of control rods is typically required in such reactors. The control drive system thus must operate in an extremely limited space thus complicating the reactor control mechanisms.
The present invention addresses the aforementioned problems encountered in a small nuclear reactor such as of the liquid metal fast breeder type by providing, in one embodiment, a single control element which is located outside of the reactor core and is thus easily accessible. Another embodiment employs a plurality of control elements which are located outside of the reactor core, are easily manipulated, and which provide a high degree of control over the reaction rate within the reactor core.